2002 Monte Carlo SS. Here yet again. Now to start off, the car has a hollowed cat on it. The people I bought it from, hollowed the cat out. Now just one day, I was driving it and it was absolutely perfect and then it sputtered down when I gave it some throttle and threw a check engine light for P0420. Still ran pretty good unless I gave it like 50% throttle or higher. Then I went to start it after it sat for a few hours and it started missing bad and eventually led to the P0300 popping up. Now it misses absolutely horrible. When going highway speeds it runs rather good, but going below 40mph or at idle, it’ll miss horribly. I had just put new spark plugs in like 200 miles ago. I’ve ran Lucas fuel treatment in it as well and the fuel filter has been changed maybe 600 miles ago. Any ideas?